Fix precision-recall curve based computations for float target

[SkafteNicki]

What does this PR do?

Fixes #1641
Some missing dtype casting.

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1642 (e67148b) into master (685391d) will decrease coverage by 38%.
The diff coverage is 100%.

❗ Current head e67148b differs from pull request most recent head 9209c12. Consider uploading reports for the commit 9209c12 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1642     +/-   ##
========================================
- Coverage      89%     51%    -38%     
========================================
  Files         228     228             
  Lines       12363   12363             
========================================
- Hits        10945    6304   -4641     
- Misses       1418    6059   +4641